1. Field of the Invention
This invention relates to endless power transmission belts having regularly longitudinally spaced blocks on an endless center belt and, more particularly, to structure for connecting the blocks to the center belt to provide a long lasting belt with high force transmission capabilities.
2. Background Art
Toothed power transmission belts are commonly used in environments wherein the belts will be subjected to heavy loading. It is known to construct a belt of this type by providing a center belt section and attaching blocks on oppositely facing surfaces of the center belt section to define equidistantly spaced drive teeth.
One known method of connecting the blocks to the center belts involves the use of bolts or rivets which are directed through an upper and lower block pair and a center belt, which is captively held between the blocks in the pair. This arrangement has accounted for secure holding of the blocks to the center belt. Belts of this construction have exhibited relatively good strength in, for example, high load transmissions.
One problem with the above prior art belt construction is that the bolts or rivets, which secure the blocks to the center belt, must pass through the center belt. Holes in the center belt may appreciably weaken the center belt. A still further problem with the above prior art belt construction is that the fabrication thereof requires a large number of assembly steps. Holes must be pre-bored in the blocks and center belt. This may be a labor intensive process that requires great precision. Further, the bolts and rivets must be secured in separate steps. Both operations are sufficiently involved that the cost of assembly may be relatively high.
To avoid the above problem of center belt weakening, the assignee herein developed a belt construction in which the blocks are rigidly fixed to a center belt without any penetration of the center belt by the block fasteners. This structure, which is disclosed in Japanese Patent Publication No. 1-261138, has proven to be economical to manufacture and successful in use.
While the block fasteners in this last belt construction do not weaken the center belt, they are fastened such that the surface area of the pulley-engaging side surfaces of the belt blocks is reduced. More particularly, the fasteners are in the form of a wire which is wrapped in a direction transversely to the belt length within a groove/recess defined by an undercut continuously around the belt in both the upper and lower blocks.
The undercut at the pulley-engaging side surfaces of the blocks has two detrimental effects. First of all, the undercut reduces the contact area between the side surfaces and a cooperating pulley and thereby diminishes the force which the belt can transmit through friction between the block side surfaces and a cooperating pulley. Slippage of the belt may also occur which abrades the side surfaces of the belt blocks to reduce belt life.
Secondly, the undercut reduces the thickness of the blocks to diminish the block rigidity. This affects the force transmission capabilities as well as reducing the length of the useful belt life.
It is further known to construct center belts from a polymeric material that has load-carrying cords embedded therein. The load-carrying cords are preferably formed by spirally wrapping a cord made of aramid fiber, polyester fiber, or the like.
Because the cords are wrapped in a spiral path, the length of the cords is not fully aligned with the length of the belt. The stability of the belt, during running, is thus adversely affected. Vibrations and/or shocks may be induced to the system as a result of this construction. Another problem with spirally wrapped cords is that the tension applied to the belt may only be transmitted through some of the cords. Inconsistency in belt performance, from one to the next, may result. The life of the belt may thus be unpredictable.
The above problem is aggravated when a fastener is extended, through the center belt and mounting blocks thereon. In belts utilizing bolts, rivets or the like, the center belt is typically pre-drilled. During this operation, one or more of the load-carrying cords may be partially cut or altogether severed. As a consequence, the strength of the belt in the longitudinal direction is diminished. Because of the spiral arrangement of the load-carrying cords, the weakening due to the center belt bore is not only localized lengthwise parallel to the bores but also extends laterally outwardly a significant distance from the bores.
Another problem that is encountered with prior art belts results from the connection of the blocks with multiple fasteners. The belt may be made undesirably wide strictly to accommodate the multiple fasteners.